The present disclosure relates to image forming apparatuses and particularly relates to a technique for improving image quality and preventing toner scattering in an image forming apparatus employing a belt transfer system in which a toner image on the surface of a photosensitive drum is transferred by a belt type transfer section.
Electrophotographic image forming apparatuses, such as a multifunction peripheral, a printer, a copier, and a facsimile device, include those employing a belt transfer system in which a toner image on the surface of a photosensitive drum (image carrier) is transferred by a belt type transfer section. The image forming apparatus with the belt transfer system is configured to induce electric charges on the surface of a transfer belt to electrostatically attract a recording paper sheet to the transfer belt, convey it in this state, and allow a toner image formed on the surface of a photosensitive drum to be transferred to the recording paper sheet in a nip between the transfer belt and the photosensitive drum.
This belt transfer system is excellent in terms of separability of the recording paper sheet from the photosensitive drum, stability of sheet conveyance, and image transferability but may cause separation discharge when the recording paper sheet after the transfer of the toner image thereto separates from the transfer belt. The separation discharge may scatter toner on the surface of the recording paper sheet, which results in a problem of the occurrence of an image defect (electrostatically toner-scattered image) and a problem in that scattered toner is accumulated in the apparatus to contaminate the apparatus interior. Furthermore, the recording paper sheet is strongly charged to the same polarity as the toner, which presents a problem of the occurrence of electrostatic offset in the fixing process.
As a solution to the above problems, a technique (Technique A) is known in which the image-forming surface of the recording paper sheet is exposed to an AC discharge at a position just short of where the recording paper sheet separates from the transfer belt, thus eliminating static electricity from the sheet to prevent separation discharge during separation of the sheet from the belt and the AC bias value is controlled to reduce the contamination of an AC discharger. As another solution, a technique (Technique B) is known in which a high-resistance, sheet-shaped transfer exit guide plate is provided downstream of the transfer belt in the direction of conveyance of the recording paper sheet to eliminate the static electricity of the sheet separated from the transfer belt using the guide plate and thus prevent separation discharge of the sheet.